Layer-by-layer solid imaging techniques, including stereolithography and selective laser sintering, have been used to produce models, mold patterns, and near net shape production parts. Stereolithography provides high accuracy and excellent surface finish, but does not normally allow for production of objects in engineering materials such as nylon or ABS. Selective laser sintering can produce objects in nylon or polycarbonate but cannot produce objects of full density, and generally produces objects having low surface quality.
U.S. Pat. No. 5,354,414 describes imagewise formation of an integral object which can be separated from the non-imaged regions. Processes of brazing, soldering and welding are disclosed for producing the integral object.
U.S. Pat. No. 4,575,330 describes generation of three-dimensional objects by creating a cross-sectional pattern of the object on the surface of a fluid medium. The physical state of the medium is altered to form successive cross-sectional layers and provide a step-wise laminar buildup of the desired object. As an example of a suitable change in physical state, the patent discloses free-radical curing of acrylate.
U.S. Pat. No. 5,474,719 describes a solid imaging process wherein high viscosity liquids are viscosity-reduced during coating and allowed to increase in viscosity during imaging steps. In particular, the compositions contain a photohardenable monomer and a photoinitiator, and the photohardening methods suggested involved free-radical polymerization, cationic polymerization, anionic polymerization, condensation polymerization, addition polymerization, and the like.
U.S. Pat. No. 4,938,816 describes a process for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The bulk density of the powder is increased prior to sintering or melting the powder by exposure. Attaining high bulk density of powders for sintering is difficult, however, and requires the use of substantial pressures.
It is known in the art that certain advantages are associated with providing at least one above solidus temperature component in a dispersion to be sintered. The advantages are, for example, improved wetting between the components, reduction of friction between the components to allow for greater densification, capillary forces which draw the components together and drive densification, and greater molecular diffusion between components during sintering. See, for example, R. M. German, Liquid Phase Sintering, Plenum Press, New York, 1985; and Eremenko et al, Liquid Phase Sintering, Plenum Publishing Corp., New York, 1970. These disclosures do not teach imagewise layer-by-layer formation of three-dimensional integral articles.